1. Field of the Invention
This invention relates to a method and an apparatus for detecting a direction and a magnitude of motion of a moving object, or namely, for detecting motion vectors, in a digitized video signal.
2. Description of Related Art
The television system is one of image telecommunications for transmitting visual information such as various scenes and pictures to remote viewers using optical and electrical means. The television system fundamentally converts on its transmitter side a three dimensional image into a two-dimensional (plain) image by a lens, transmits optical energy of respective parts of the image in a form which has been converted into a one-dimensional electrical signal, and composes it again on the system's receiving side to a two-dimensional image to form a picture. In television, plural pictures are consecutively displayed, thereby reproducing movement of objects in the picture. Each of the plural pictures being continuously transmitted is called a frame.
The image and the picture composed from the image are assumed as an assembly of tiny blinks (pixels) having luminance valves that are different from one another. In such a television system, a picture is divided into matrixes of tiny blinks on the transmitter side, and then, the luminance of each blink in the picture is converted into an electrical signal according to a fixed order from the left to the right and from the top to the bottom and is transmitted to the receiving side. The electric signal is sequentially composed on the receiving side as pictures according to its order as sent from the transmitting side. Such regularly decomposing or composing of one picture is called scan or scanning.
Scanning is typified by a sequential scanning, or a non-interlace scanning, and by an interlaced scanning. In non-interlaced scanning, one picture is completed by scanning sequentially from the top to the bottom of the picture. In the interlaced scanning, scanning of a picture is completed by scanning twice, namely, by first scanning the picture with space between scan lines from the top to the bottom and next scanning the between spaces at which the first scanning lines did not proceed. The interlaced scanning can be said to form one fine picture, or the frame, by substantially superimposing plural rough pictures. Each of the plural rough pictures is called a field. The interlaced scanning can reduce the CFR (critical flicker frequency) of the picture without impairing its resolution. Therefore, most television cameras now use the interlaced scanning.
A motion vector indicates a magnitude and a direction of a moving object in a scene. This motion vector is used, for example, for interframe coding during a high efficiency coding of a television signal and for a field interpolation for a field number conversion as a required process for television standard conversions. A method, generally called a pattern matching method, in which motion vectors are detected using similarity of signal patterns between frames, has been known as a motion vector detecting method. Such a method has been disclosed in Japanese Unexamined Patent Publication Nos. Showa 55-162,683 and Showa 55-162,684. In addition to it, a method, generally called an iterative gradient method, in which motion vectors are presumed for example, from physical correspondence of a signal gradient in a frame or an interframe signal difference value, has been known and disclosed in Japanese Unexamined Patent Publication No. Showa 60-158,786. A method using initial vectors among the iterative gradient methods improves its detection accuracy of detection of the motion vectors, and is disclosed in Japanese Unexamined Patent Publication Nos. Showa 62-206,980 and Heisei 4-78,286.
Even with such a motion vectors detecting method in which shift vectors are calculated based on a detected motion vector such as an initial vector and of the initial vector and the shift vector are summed to obtain a true motion vector, however, detection errors may occur due to initial vectors' incapability of response to a sudden change of the motion vector when the object suddenly moves from its still state or when it suddenly stops from its moving state. In particular, with a method having low accuracy of detection of the motion vectors, likewise the gradient method, sudden changes of the motion vectors induce occurrences of detection errors.
For example, as shown in FIG. 1(A), where an object M which had been still in the picture of the previous field travels in the present field, a background image of the moved object M comes to appear at the hatching area in FIG. 1(B). If the background image is the still picture or has no change between two successive fields, the motion vector for this area must be null. However, the detection result of the motion vector cannot in fact be null. As a result, image distortion may occur in an interpolated image in this area. Although the prospective initial vectors of a block m1 showing the object M are produced using the detected motion vector, all the prospective initial vector becomes null because the image in FIG. 1(A) is the still picture one at all. With this gradient method to be used for such a situation, since only an area at which the image gradient exists allows the operation, detection errors may occur upon detection of motion vectors for the block m1 in FIG. 1(B).
Where a moving object m2 from the previous field shown in FIG. 2(A) to the present field shown in FIG. 2(B) in the picture is tiny and moves greatly, detection errors in detecting the motion vector may occur in the gradient method in principle. In contrast, where the background is the motion picture, or has changes between two successive fields, and a nearly still object in detecting a small area exists therein, detection errors of the motion vectors occur similarly. For example, where a TV camera takes pictures of a volley ball game and when the camera is panned along with the motion of the moving volley ball, the background moves whereas the volley ball itself makes the still image. If the movement of the background is great, it affects the data necessary when the still image of the volley ball is corrected using the motion vectors, resulting in occurrences of image distortions.
FIG. 3 shows this example, in which a small ball A is viewed as a still picture portion and a background B moves with a motion amount V. Where the ball size is almost equal to a block size for motion vector detection (in FIG. 3, a block size is set to 8 pixels.times.8 lines), the full magnitude of the initial vector in the block A becomes a motion amount V for it uses the already detected motion vector. Consequently, a shift vector of -V magnitude must be generated to make the block A still, or namely, to make the motion vector null. However, if the magnitude V is so great, the shift vector of -V does not tend to be generated where, as with the gradient method, the motion vectors are detected with low accuracy, so that the motion vector of A does not become null.
Furthermore, regarding an image having a small vertical correlation likewise a crosshatching pattern, a problem arises in which detection errors easily occur when vertical motion vector Vy is detected. Moreover, where, likewise a TV standard converting apparatus, an interpolated image is produced using the detected motion vectors, the detection errors of such motion vectors produce image distortions.